This invention relates to negative electrodes for use in batteries (e.g., lithium electrodes for use in lithium-sulfur batteries). More particularly, this invention relates to methods of forming alkali metal electrodes having a thin glassy or amorphous protective layer.
In theory, some alkali metal electrodes could provide very high energy density batteries. The low equivalent weight of lithium renders it particularly attractive as a battery electrode component. Lithium provides greater energy per volume than the traditional battery standards, nickel and cadmium. Unfortunately, no rechargeable lithium metal batteries have yet succeeded in the market place.
The failure of rechargeable lithium metal batteries is largely due to cell cycling problems. On repeated charge and discharge cycles, lithium xe2x80x9cdendritesxe2x80x9d gradually grow out from the lithium metal electrode, through the electrolyte, and ultimately contact the positive electrode. This causes an internal short circuit in the battery, rendering the battery unusable after a relatively few cycles. While cycling, lithium electrodes may also grow xe2x80x9cmossyxe2x80x9d deposits which can dislodge from the negative electrode and thereby reduce the battery""s capacity.
To address lithium""s poor cycling behavior in liquid electrolyte systems, some researchers have proposed coating the electrolyte facing side of the lithium negative electrode with a xe2x80x9cprotective layer.xe2x80x9d Such protective layer must conduct lithium ions, but at the same time prevent contact between the lithium electrode surface and the bulk electrolyte. Many techniques for applying protective layers have not succeeded.
Some contemplated lithium metal protective layers are formed in situ by reaction between lithium metal and compounds in the cell""s electrolyte which contact the lithium. Most of these in situ films are grown by a controlled chemical reaction after the battery is assembled. Generally, such films have a porous morphology allowing some electrolyte to penetrate to the bare lithium metal surface. Thus, they fail to adequately protect the lithium electrode.
Various preformed lithium protective layers have been contemplated. For example, U.S. Pat. No. 5,314,765 (issued to Bates on May 24, 1994) describes an ex situ technique for fabricating a lithium electrode containing a thin layer of sputtered lithium phosphorus oxynitride (xe2x80x9cLIPONxe2x80x9d) or related material. LIPON is a glassy single ion conductor (conducts lithium ion) which has been studied as a potential electrolyte for solid state lithium microbatteries that are fabricated on silicon and used to power integrated circuits (See U.S. Pat. Nos. 5,597,660, 5,567,210, 5,338,625, and 5,512,147, all issued to Bates et al.).
In both the in situ and ex situ techniques for fabricating a protected lithium electrode, one must start with a smooth clean source of lithium on which to deposit the protective layer. Unfortunately, most commercially available lithium has a surface roughness that is on the same order as the thickness of the desired protective layer. In other words, the lithium surface has bumps and crevices as large as or nearly as large as the thickness of the protective layer. As a result, most contemplated deposition processes cannot form an adherent gap-free protective layer on the lithium surface.
Thus, lithium battery technology still lacks an effective mechanism for protecting lithium negative electrodes.
The present invention provides an improved method for forming active metal electrodes having protective layers. Active metals include those metals that can benefit from a protective layer when used as electrodes. The method involves plating the active metal between a protective layer and a current collector on an xe2x80x9celectrode precursor.xe2x80x9d The electrode precursor is formed by depositing the protective layer on a very smooth surface of a current collector. Because the surface on which the protective layer is deposited is very smooth, the protective layer has a higher quality than when deposited directly on thick lithium metal. During plating, active metal ions move through the protective layer and an active metal layer plates onto the surface of the current collector. The resulting structure is a protected active metal electrode. To facilitate uniform plating, the electrode precursor may include a xe2x80x9cwetting layerxe2x80x9d which coats the current collector.
One aspect of the invention provides a method of fabricating an alkali metal electrode, which method may be characterized by the following sequence: (a) providing an alkali metal electrode precursor to an electrochemical cell, which electrode precursor includes a current collector and a glassy or amorphous protective layer forming a substantially impervious layer which is a single ion conductor conductive to ions of an alkali metal; and (b) plating the alkali metal through the protective layer to form a layer of the alkali metal between the current collector and the protective layer to form the alkali metal electrode. Preferably, the alkali metal electrode precursor also includes a wetting layer located between and adherent to the current collector and the protective layer. The wetting layer facilitates even deposition of the alkali metal on the current collector. Note that current collectors are typically inert to the alkali metal and therefore do not provide good plating surfaces. Often the alkali metal plates unevenly over the surface. In a preferred embodiment, the wetting layer either (i) intercalates alkali metal ions conducted by the single ion conductor or (ii) alloys with the alkali metal having ions conducted by the single ion conductor.
The alkali metal may be plated in situ or ex situ. In the in situ case, a battery is assembled from the electrode precursor and other battery elements including an electrolyte and a positive electrode. The electrode precursor is then converted to an alkali metal electrode by an initial charging operation in which lithium plates from the positive electrode. The battery may be either a primary or secondary battery. Prior to the plating step, such batteries do not contain free alkali metal. This allows for safe transportation and long shelf life. Only when a battery cell is ready for use is it charged for the first time to form the alkali metal electrode. Only then does it contain free alkali metal.
In the ex situ case, the electrode is formed in an electrolytic cell that is separate from the battery in which it is ultimately assembled. Thereafter the electrode is removed from the electrochemical cell and assembled into a battery.
The present invention also relates to alkali metal electrode precursors which may be characterized by the following features: (a) a current collector; (b) a glassy or amorphous protective layer forming a substantially impervious layer which is a single ion conductor conductive to ions of an alkali metal; and (c) a wetting layer located between and adherent to the current collector and the protective layer. As mentioned in the method aspect of this invention, the wetting layer either (i) intercalates alkali metal ions conducted by the single ion conductor or (ii) alloys with the alkali metal having ions conducted by the single ion conductor.
The current collector is typically a layer of metal such as copper, nickel, stainless steel, or zinc. Alternatively, it may be a metallized plastic sheet or other metallized insulating sheet. If the wetting layer material alloys with the alkali metal, it may be silicon, magnesium, aluminum, lead, silver, or tin, for example. If the wetting layer intercalates ions of the alkali metal, it may be carbon, titanium sulfide, or iron sulfide, for example.
If the alkali metal is lithium, the protective layer should be conductive to lithium ions. Examples of suitable lithium ion conducting protective layer materials include lithium silicates, lithium borates, lithium aluminates, lithium phosphates, lithium phosphorus oxynitrides, lithium silicosulfides, lithium borosulfides, lithium aluminosulfides, and lithium phosphosulfides. Specific examples of protective layer materials include 6LiIxe2x80x94Li3PO4xe2x80x94P2S5, B2O3xe2x80x94LiCO3xe2x80x94Li3PO4, LiIxe2x80x94Li2Oxe2x80x94SiO2, and LixPOyNz (LIPON). Preferably, the protective layer has a thickness of between about 50 angstroms and 5 micrometers (more preferably between about 500 angstroms and 2000 angstroms). Preferably, the protective layer has a conductivity (to an alkali metal ion) of between about 10xe2x88x928 and about 10xe2x88x922 (ohm-cm)xe2x88x921.
As noted, the electrodes and electrode precursors of this invention may be assembled into alkali metal batteries. In a specific embodiment, the invention provides alkali metal batteries that may be characterized by the following features: (a) a positive electrode comprising a source of mobile alkali metal ions on charge; (b) a precursor to an alkali metal negative electrode as described above; and (c) an electrolyte. Preferably, the alkali metal is at least one of lithium and sodium. The electrolyte may be liquid, polymer, or gel. In a particularly preferred embodiment, the positive electrode includes at least one of sulfides of the alkali metal, polysulfides of the alkali metal.
Examples of suitable primary batteries include lithium manganese dioxide batteries, lithium (CF)x batteries, lithium thionyl chloride batteries, lithium sulfur dioxide batteries, lithium iron sulfide batteries (Li/FeS2), lithium polyaniline batteries, and lithium iodine batteries. Examples of suitable secondary batteries include lithium-sulfur batteries, lithium cobalt oxide batteries, lithium nickel oxide batteries, lithium manganese oxide batteries, and lithium vanadium oxide batteries. Other batteries employing active metals other than lithium may be employed as well. These include the other alkali metals and alkaline earth metals.
These and other features of the invention will be further described and exemplified in the drawings and detailed description below.